Hook up the updated `WikiLinkFilter` to the wiki controllers.

- Need to pass in a `page_slug` to the filter, so it can rewrite based
on the current page (all links are rewritten to the level of the app root).
- The earlier `markdown_preview` endpoint was at the level of the wiki.
We need to know the current page (for rewriting, as above), so this
commit moves the endpoint to the level of a wiki page.
- Fix all tests